General Background and State of the Art
This invention relates to variable-property products, and methods for making such products.
A variable-property product is an article that has varied material properties such as stiffness and/or surface frictions and/or elasticity. As an example, medical catheters may be constructed with variable stiffness properties. These catheters have been developed to have different degrees of flexibility throughout their length to satisfactorily meet the requirements of flexibility and also stiffness for manipulation.
Variable stiffness products are currently made with a special extrusion process, called Interrupted Layer Co-extrusion (herein ILC), for making long articles with variable-stiffness properties. U.S. Pat. Nos. 5,533,985 and 5,622,665 describe the ILC process in detail. The contents of each of these patents and all other patents mentioned in this disclosure are hereby incorporated by reference in their entireties.
The prior art processes use a xe2x80x9ctip-and-diexe2x80x9d assembly to make long articles such as tubing. A xe2x80x9cnon-pressure-diexe2x80x9d set-up requires the tip to extend to the end of the co-extrusion head, flush with, or beyond the face of the die. In a xe2x80x9cpressure-diexe2x80x9d set-up, the tip is recessed in relation to the die.
In the ILC process, extruded products such as tubing and electrical cables are manufactured with commonly used rotating extruders. The polymer streams that come from the extruders are then fed through modulators. Usually there are two modulators for every variable-stiffness material stream (polymer stream). The main modulator is to direct the polymer flow and the relief modulator is to reduce the residual flow. In the prior art process, first, the polymer is directed to the co-extrusion head to form the product. Next, the polymer is directed to bleed to the floor as the waste material.
Although the ILC process works well in making some variable-material products, there are a number of problems. In the ILC process, roughly one pound of scrap is generated for every pound of catheters made for a two-polymer system in some cases. The bleeding scrap rate worsens to four pounds of scrap per one pound of products, in some five-polymer systems envisioned. This compares with the normal scrap rate of only 0.1 pound of scrap per pound of goods in a typical extrusion operation. Much of the scrap in an ILC process is generated through bleeding.
Further problems in the ILC process are poor yield for some products, due to such things as modulator breakdowns, solidifying of bleeds, etc. Fluctuations in bleeding can also lead to less than desired repeatability. With hot polymer bleeding to the production floor at all times, the operation of the ILC process is quite messy.
For some more critical products, the transition section from a two-polymer system is not pliable enough. Yet, it is impractical to use more than three polymers in the ILC process, because that would require the use of too many unreliable modulators.
In prior art processes for developing variable stiffness extruded products such as tubing, the products are prone to collapsing upon bending stress, also known as xe2x80x9cchinking,xe2x80x9d more easily at certain points in the transition sections between the different materials.
The transitional chinking problem is due to a thin, stiff tail layer of material in the transition section. The thin stiff layer is the natural result of the polymer flow in the head of the extrusion device. In the prior art, when the stiffness ratio between the stiff polymer and the flexible polymer exceeds a certain value, the composite structure tends to chink. This chinking phenomenon is especially acute for thin walled products, such as catheters.
Another drawback of the prior art processes is the time wasting practice in changing the transition length of a product. To change the transition length, the line must be shut down, the parts changed, and the line started up again to see if the new transition length is satisfactory. If the transition length is unsatisfactory, the same steps have to be repeated over again until the right length is found. This is a very time consuming practice.
In the prior manufacturing process for variable-material products, one of the main drawbacks is that an excessive amount of core scrap is generated. Most core scrap is generated in the purge duration of a manufacturing cycle. In the purge duration, the residual stiff material is purged out by the flexible material so the next production duration of the manufacturing cycle can begin.
To reduce core scrap, the flexible polymer flow rate should be increased to quickly purge out the residual stiff polymer, and the core movement should be significantly slowed down during purging, to use less core material. Unfortunately, this cannot be effected in the prior art. It is well known in extrusion processes, such as the ILC process, for making elongated articles such as tubing or electrical wires, that the speed of the core movement has to closely match the polymer flow rate. In other words, when the polymer flow rate is increased, the core movement has to speed up. The polymer flow rate cannot be increased while simultaneously slowing down the core movement. This is especially true when thin polymer walled products, such as catheters, are involved.
The problem in the prior art exists because when a certain tip and die assembly, such as a pressure-die set-up, is used to make thin polymer walled products, the core speed largely controls the polymer flow rate in the main channel. Simply, the higher the core speed, the more polymer it drags out through the die opening, and the more polymer flows through the main channel.
One aspect of the present invention is to develop a method of forming variable-property material such as tubing, which eliminates the vast amount of bleeding waste of polymers that occurs in prior art processes.
Another aspect of the present invention to make pliable products that contain three or more polymers.
Yet another aspect of the present invention is to develop a process better suited for repeatability, better consistency and fewer breakdowns.
A further aspect of the present invention is to develop a process with improved general cleanliness.
These and other aspects are achieved by the process of the present invention which, in accordance with a broad structural aspect of the invention, includes material injectors, instead of commonly used extruders, used in the prior art ILC process to push material through a tip-and-die head. Prior art extruders deliver steady streams of polymers at constant rates while injectors deliver intermittent shots of polymers at controlled pressures.
Two or more injectors take turns making polymer xe2x80x98shotsxe2x80x99 to maintain a continuous polymer stream. In one form of the invented polymer injector, high pressure is exerted at the back of a reciprocating screw which forces the screw to move forward through the injector to send a shot of material, such as molten polymer, out of the injector. The pressure at the back of the screw is then reduced to eliminate the residual flow. The screw then rotates and slowly moves back to replenish the molten polymer in the injector.
The present invention includes the novel use of a singular flow controller for all material streams coming from the injectors.
In one form of the invention for making variable-property products, such as variable-stiffness products, the present invention uses two or more intermittent acting injectors to make continuous articles such as tubing, cables, wires, etc. Preferably, the present invention uses a tip-and-die assembly such as a pressure-die setup in combination with the injectors, drastically reducing the inherent problem of lack of precise control over polymer flow rates for injectors.
In prior art ILC processes, half of the modulators are used for reducing residual flows coming from the extruders. In the present invention, half of the modulators are eliminated in the pressure reduction step of the injectors. The other half of the modulators is replaced by a singular flow controller.
One other aspect of the present invention is to provide a method of forming variable-property material such as tubing which eliminates the chinking that occurs in the prior art processes.
Another aspect of the present invention is to provide a method of forming variable-property material such as a medical catheter which has a more flexible distal section.
It is a further aspect of the present invention to provide a process better suited for changing the transition length of a product, in a less time consuming manner than in the prior art.
These and other aspects are achieved by the present invention which, in accordance with a broad structural aspect of the invention, includes a blending feature in the head of the device which intensely mixes material together as it passes through it. The product that results from the present invention has a transition section comprised of a blend of different types of material.
In an exemplary embodiment for making variable-property products, such as variable-stiffness products, the present invention uses a blending feature in the head of a device to make products such as tubing or electrical wires, where the variable property materials are intensely mixed together to form the new type of transition section shown.
In another aspect of the present invention, a method of forming a variable-property product such as tubing, in a tip and die assembly, allows for a much-increased material flow during the purging process than in prior art.
In another aspect of the present invention, a method of forming a variable-property product such as tubing, in a tip and die assembly, allows for a much increased material flow and a much slowed core speed at the same time, during the purging process, to reduce core scrap.
In yet another aspect of the present invention, a method of forming a variable-property product such as differential-stiffness tubing, in a tip and die assembly, allows for a much more flexible distal section in the product.
In a further aspect of the present invention, a method of forming a variable-property product such as variable stiffness tubing, using a tip and die assembly, allows for the elimination of the thin residual stiff layer that is usually left on the surface of the flexible section of a variable stiffness tubing.
These and other aspects are achieved by the present invention which, in accordance with a broad structural aspect of the invention, includes a diverting feature in the head of a tip and die assembly which allows for a simultaneous increase in material flow and a decrease in core speed.
Other objects and advantages of the present invention will become more apparent to those persons having ordinary skill in the art to which the present invention pertains from the foregoing description taken in conjunction with the accompanying drawings.